A cold cathode fluorescent light (CCFL) has been increasingly used as a back light source of a liquid crystal display (LCD) of, for example, a notebook PC and a TV set. Such CCFL has substantially the same high efficiency and life as a usual hot cathode fluorescent light, without using a filament of a hot cathode fluorescent light.
In order to start up and operate the CCFL, high AC voltages are required. For example, a startup voltage of about 1000 V and an operating voltage of about 600 V (in rms value) are required. (The voltages will be hereinafter given in rms value.) These high AC voltages are generated from a DC power source provided in, for example, a notebook PC and a liquid crystal TV set, using an inverter.
An inverter for supplying AC power to a CCFL, configured to attain high power conversion efficiency is disclosed in Japanese Patent Application Laid Open No. 10-50489 (referred to as Patent Document 1). This inverter has a first semiconductor switch connected in series with the primary winding of a transformer, a second semiconductor switch and a capacitor which are connected in series to each other and connected in parallel with the primary winding of the transformer, and a coupling capacitor connected in series with a load and with the secondary winding of the transformer. The primary current of the transformer is fed back to a control circuit where the voltage indicative of the current is compared with a reference voltage to form a control signal for performing on-off control of the first and second semiconductor switches to thereby supply required AC power to the load.
A full bridge (or H-bridge) type inverter for CCFL that utilizes four semiconductor switches is disclosed in Japanese Patent Application Laid Open No. 2002-233158 (referred to as Patent Document 2). This inverter includes a transformer having a primary winding connected in series with the output end of an H-bridge, and a secondary winding to be connected to a load. Of the four semiconductor switches constituting the H-bridge, a first set of two semiconductor switches establishes a current path through the primary winding in a first direction, while a second set of two semiconductor switches establishes a current path through the primary winding in a second direction. The inverter generates a control signal consisting of pulses having a fixed pulse width and controlled relative pulse positions by feeding back the current flowing through the secondary winding to a control circuit for comparison with a reference voltage. The control signal controls the semiconductor switches of the H-bridge to regulate the power supplied to the load. The secondary voltage of the transformer is detected to protect the transformer from an over-voltage.
Another inverter for providing power to a CCFL is also disclosed in Japanese Patent Application 2002-221701 (referred to as Patent Document 3). This inverter has a power supply unit that undergoes controlled intermittent operation, wherein the ratio of the ON- to OFF-duty periods of the intermittent operation is regulated by pulse-width modulation (PWM) based on a detection signal indicative of the current flowing through the CCFL so as to maintain the current at a predetermined level.
In the inverter of Patent Documents 1 and 2, the ON duty periods (conduction period) of the semiconductor switches are controlled so as to maintain the load current at the predetermined level. To reduce power supplied to the load, the widths of the control pulses switching on the semiconductor switches may be reduced. However, there is a limitation in the reduction of the pulse widths to provide small power to the load in a stable condition. Hence, it is difficult to downwardly extend the lower limit of dimmer control of the load in the form of a CCFL.
Moreover, it is difficult to perform fine dimmer control using the inverter of Patent Document 3, since the inverter controls only the ON-OFF ratio of the duty periods. The inverter has a further problem in that it requires a complex slow-start arrangement to suppress hamming of the transformer and overshooting of the output current caused by the intermittent operation.
It is, therefore, an object of the invention to provide an inverter comprising a transformer having a primary winding connected to a semiconductor switch circuit and a secondary winding connected to a load, the inverter being capable of
supplying a wide range of power to the load via the switch circuit undergoing intermittent operation controlled by pulse width modulation (PWM) for constant-current operation; and
making a slow start at the time of startup and at each phase of the intermittent operation by means of a simple circuit arrangement, thereby considerably reducing hamming of the transformer while preventing overcurrent to the load.
It is another object of the invention to provide a controller IC for use with such inverter stated above.
It is still another object of the invention to provide an electronic device equipped with such an inverter as stated above and a light emitting device driven by the inverter.